The present invention relates generally to motor drive systems operable to control tension in a web coiling operation and more particularly to such systems which include compensation for inertial effects of the system when it is accelerating and decelerating.
In many industries such as paper making and metal rolling, one of the final operations is the winding of the web material into a coil or reel so that it may be easily transported for additional fabrication or processing. One of the most common motor drives used in web coiling operations employs the shunt wound, direct current (d.c.) motor having controlled excitation of both the armature and field windings of the motor. In these systems, tension in the web is controlled by controlling the torque supplied to the coiling reel. This torque control is normally achieved by controlling the armature winding current. In these systems, it is customary to include some form of compensation for system inertia so that the web is properly coiled and does not stretch or break when the system is accelerating or decelerating.
Two system inertias are known to be of primary importance. The first of these varies directly as the square of the ratio (R) of the radius of the coiled web material to the maximum radius of the final coil. The second inertia of concern is that occasioned by the rest of the moving parts of this system excepting the web material and this varies inversely with the square of the ratio R. In mathematical terms, the compensation to the armature current is expressed as: EQU I.sub.A-CO =K.sub.1 R.sup.2 +K.sub.2 /R.sup.2, (1)
wherein, the terms K.sub.1 and K.sub.2 are system constants which, although are capable of being mathematically calculated, are normally derived by empirical methods for the particular system in question.
In a common prior art method, compensation for the inertias is achieved by a straightforward electrical application of the above formula. That is, the maximum radius is defined and then the instantaneous actual radius of the coil is determined from the relationship between the linear speed of the web material and the rotational speed of the coil or reel. This requires suitable speed measuring devices such as tachometers for each component; for example, one tachometer associated with the web feed rollers and another with the coil. Having derived the two relative speeds and with the knowledge of the two constants K.sub.1 and K.sub.2, the compensation armature current I.sub.A-CO is then derived using analog dividers, multipliers and adders in, as stated, a pure electrical implementation of formula (1).
This prior art system functions very well and provides very accurate compensation. It is also extremely expensive to properly implement. As indicated, two speed measuring devices are required and the analog circuitry to perform the indicated calculations all result in an expensive system, especially when the high quality components necessary for accurate computations and compensation are used.